This invention relates to an electronic detection system. Such systems are much used in department stores to detect shoplifting. For this purpose the goods to be protected are provided with a detection plate or responder, which normally is removed at the cash desk. Furthermore, at the exits of the shop an electromagnetic field is generated, to which a responder carried through this field reacts. This reaction, which may be either principally energy absorption or principally energy transmission, can be detected, so that an indication can be obtained of the fact that merchandise still provided with a responder is carried through the field.
Such a system, which is based on energy absorption by the responder, is known, for example, from U.S. Pat. No. 3,500,373.
Generally speaking, such a system is suitable for detecting the passage of goods, animals or persons provided with a responder through a detection zone. If identification of the kind of goods, an animal or a person, is desirable, the reaction of the responder may be a coded signal.
Systems of the kind described are particularly suitable for use in detecting theft in shops. In such systems, a responder is attached to articles to be safe-guarded, which responder is removed at the cash desk upon payment. At the shop's exits, an interrogation zone is created so that, if goods still provided with a responder pass the interrogation zone, this can be detected.
The known anti-shop-lifting systems are all intended for safe-guarding large numbers of goods. This means that large numbers of responders are required. This in turn means that price of the responders must be low, which leads to a structurally and electrically simple responder, often just consisting of a resonance circuit embedded in a detection plate, or of a strip of magnetic material.
Owing to the simplicity of such responders, it is virtually inevitable that electrical processes similar to those occurring in the responder also occur in other articles which pass the interrogation zone. This may create a false alarm, which is highly undesirable. Spurious electrical and radio signals can also cause such false alarms.
It is true that the chance of false alarms can be reduced by special features in the transmitter generating the interrogation field and/or the receiver receiving the signals from the responder in a system based on transmission, but this is also accompanied by a reduction in detection sensitivity.
Accordingly, known systems still leave much to be desired either in the field of suppressing false alarm, or in the field of detection sensitivity.
This problem could be solved by using a more sophisticated responder in which an electronic process takes place, which does not occur "in nature". Such a responder would also be more expensive than conventional responders.
A higher cost price of the responders is acceptable, if the articles to be safe-guarded, too, are relatively valuable.
There is accordingly a need for a reliable system which is in particular suitable for use in shops in which goods with a relatively high value are displayed. Examples of such shops are radio and television shops, jewellers, expensive clothes boutiques, etc. Other uses are also possible.